A bone-conduction speaker transmits an audio signal through a skull's vibration, not through air vibrations. Consequently, the bone-conduction speaker is substantially free from an external environmental noise. Due to this, the bone-conduction speaker is well adapted to any person who has difficulty in hearing, and therefore coming into increasingly wide spread use in communication instruments.
Such a bone-conduction speaker is used in an ordinary telephone set in a condition in which the bone-conduction speaker is fixedly mounted on an end portion of a main body 1 of the handset of the telephone set through a resilient board made of plastics, rubber or like materials good in damping properties in order to prevent howling from occurring, wherein the howling occurs when the bone-conduction speaker transfers its vibrations to the handset.
FIG. 9 shows a method for mounting a speaker unit of bone-conduction type on a main body of a conventional handset. In the conventional handset, a resilient board 32 is disposed between a speaker unit 33 and a main body 31 of the handset in a manner such that the resilient board 32 is brought into area contact with both the speaker unit 33 and the main body 31 of the handset. Due to this, the resilient board 32 exerts its damping effect during communication, so that, as far as possible, vibrations of the speaker unit 33 are prevented from transferring to the main body 31 of the handset.
In such a conventional method for mounting the speaker unit, it is possible to prevent howling to a certain extent due to the damping effect of the resilient board 32. However, since the resilient board is brought into area contact with both the speaker unit and the main body of the handset, it is impossible to have the speaker unit sufficiently isolated from the main body of the handset. Due to this, when a large output is issued from the speaker, it is impossible to achieve both a satisfactory prevention of howling and a satisfactory prevention of leakage of a sound. These are problems inherent in the conventional method.
See Japanese Patent documents:
    Japanese Patent Laid-open No. 2003-348208; and    Japanese Patent Application No. 2002-352000.